1. Field of the Invention
The invention relates generally to electromagnetic well logging apparatus. More specifically, antenna structures for such well logging apparatus.
2. Background Art
Electromagnetic (EM) based instruments for measuring properties of matter or identifying its composition are well known. The nuclear magnetic resonance (NMR) technique has been used to form images of biological tissues or to determine the composition of, for example, earth formations. The values of electrical conductivity biological samples or for earth formations have been obtained through the use of electromagnetic induction tools. EM propagation well logging devices are also well known, and are used for measuring basic parameters such as amplitude and phase shift of EM waves being propagated through a medium in order to determine specific properties of the medium.
Electrical conductivity (or its inverse, resistivity) is an important property of subsurface formations in geological surveys and prospecting for oil, gas, and water because many minerals, and more particularly hydrocarbons, are less conductive than common sedimentary rocks. Thus a measure of the conductivity is often a guide to presence and amount of oil, gas, or water. Induction logging methods are based on the principle that varying electric currents, due to their associated changing magnetic flux, induce electric currents.
Propagation logging instruments generally use multiple longitudinally-spaced transmitter antennas operating at one or more frequencies and a plurality of longitudinally spaced receiver pairs. An EM wave is propagated from the transmitter antenna into the formation in the vicinity of the borehole and is detected at the receiver antenna(s). A plurality of parameters of interest can be determined by combining the basic measurements of phase and amplitude. Such parameters include the resistivity, dielectric constant and porosity of the formation as well as, for example, the degree to which the fluid within the borehole migrates into the earth formation.
The transmitter antennas on induction logging instruments generate a time-varying magnetic field when a time-varying electric current is applied to them. The time-varying magnetic field induces eddy currents in the surrounding earth formations. The eddy currents induce voltage signals in the receiver antennas, whichare then measured. The magnitude of the induced voltage signals varies in accordance with the formation properties. In this manner, the formation properties can be determined.
Conventional antennas consist of coils mounted on the instruments with their axes parallel to the instrument's central or longitudinal axis. Therefore, the induced magnetic field is also parallel to the central axis of the well and the corresponding induced eddy currents make up loops lying in planes perpendicular to the well axis.
The response of the described induction logging instruments, when analyzing stratified earth formations, strongly depends on the conductive layers parallel to the eddy currents. Nonconductive layers located within the conductive layers will not contribute substantially to the response signal and therefore their contributions will be masked by the conductive layers' response. Accordingly, the nonconductive layers are not detected by typical logging instruments.
Many earth formations consist of conductive layers with non-conductive layers interleaved between them. The non-conductive layers are produced, for example, by hydrocarbons disposed in the particular layer. Thus conventional logging instruments are of limited use for the analysis of stratified formations.
Solutions have been proposed to detect nonconductive layers located within conductive layers. U.S. Pat. No. 5,781,436 describes a method that consists of selectively passing an alternating current through transmitter coils inserted into the well with at least one coil having its axis oriented differently from the axis orientation of the other transmitter coils.
The coil arrangement shown in U.S. Pat. No. 5,781,436 consists of several transmitter coils with their centers distributed at different locations along the instrument and with their axes in different orientations. Several coils have the usual orientation, i.e., with their axes parallel to the instrument axis, and therefore to the well axis. Others have their axes perpendicular to the instrument axis. This latter arrangement is usually referred to as a transverse coil, configuration.
Thus transverse EM logging techniques use antennas whose magnetic moment is transverse to the well's longitudinal axis. The magnetic moment m of a coil or solenoid-type antenna is represented as a vector quantity oriented parallel to the induced magnetic field, with its magnitude proportional to the corresponding magnetic flux. In a first approximation, a coil with a magnetic moment m can be seen as a dipole antenna due to the induced magnetic poles.
In some applications it is desirable for a plurality of magnetic moments to have a common intersection but with different orientations. For example, dipole antennas could be arranged such that their magnetic moments point along mutually orthogonal directions. An arrangement of a plurality of dipole antennas wherein the induced magnetic moments are oriented orthogonally in three different directions is referred to as a triaxial orthogonal set of magnetic dipole antennas.
A logging instrument equipped with an orthogonal set of magnetic dipole antennas offers advantages over an arrangement that uses standard solenoid coils distributed at different axial positions along the instrument with their axes in different orientations, such as proposed in U.S. Pat. No. 5,781,436.
However, it is not convenient to build orthogonal magnetic dipole antennas with conventional solenoid coils due to the relatively small diameters required for logging instruments. Arrangements consisting of solenoid coils with their axes perpendicular to the well's central axis occupy a considerable amount of space within the logging instrument.
In addition to the transmitter coils and the receiver coils, it is also generally necessary to equip the logging instrument with “bucking” coils in which the magnetic field induces an electric current in the receiver coils opposite and equal in magnitude to the current that is induced in the receiver coil when the instrument is disposed within a non-conducting medium such as, for example, air. Bucking coils can be connected in series either to the transmitter or the receiver coil. The receiver's output is set to zero by varying the axial distance between the transmitter or receiver coils and the bucking coils. This calibration method is usually known as mutual balancing.
Transverse magnetic fields are also useful for the implementation of NMR based methods. U.S. Pat. No. 5,602,557, for example, describes an arrangement that has a pair of conductor loops, each of which is formed by two saddle-shaped loops lying opposite one another and rotationally offset 90° relative to one another.
A need remains for improved antenna structures and methods for producing same, particularly for antennas having oriented magnetic dipole moments.